ADHD here: I live for finding people who know how to enjoy their hobby correctly: like you're invading a country and taking no prisoners.
I’m using a Netgear r7800 with ddwrt, with hopes to eventually move dhcp handling over to the Zima Board.
I am seriously feeling the Zima, but I just went over to Orbi Pro 6--yes, I gave in for Wifi 6 and no regrets, the coverage with just one satellite and the router is unreal. I'm trying to decide if I'll have time, but I really desperately want to learn OpenWRT; my first try was--well, there hasn't been a second one. But there will be. I picked up some (read; too many) USB Wifi dongles via rmorrow's list of linux compatible ones, so I could try and test drive a diy wifi router with it. God, that sounds fun.
The transcoding problem is one that keeps popping up. Depending on your price point, the NVIDIA SHIELD Pro (latest) can handle anything--and it is a genuinely amazing streamer and really spoils you for most of the rest--but that means it would only work when watching using that over Plex or whatever media server software you can put on it. And I think the X-Box? When I was researching during COVID, the only other all-in-one option was a full dedicated server with either Threadripper or something in that family; I think when I did the math, just for the processor, my minimum investment for 4K and Atmos/7.1 was roughly $600-$800 if I was lucky, and that's before the board and like, a sound system that does Atmos.
I know there were some other possible options with hardware, but it's been a while. If I think of anything, I'll bookmark this page to post here. Hopefully you'll find something you like and will work for you. I know exactly how frustrating it is finding a solution.
I'm watching this thread to see the recommendations. The only SBC I have seen that was designed for routers was a Pi that was on Vilros; you had to get special permission or something to even order it.